Manufacturing techniques in microelectronics and in microsystems (MEMs, MOEMs) progress in particular towards making complex volume structures, capable of better volume integration of functions of these systems.
The development of these techniques generates a change in needs for measurement and dimensional control means, exactly to give greater consideration to this volume aspect.
Optical measurement techniques, in particular based on imaging and interferometry, are widely used because they can be integrated in industrial environments and they can provide accurate information in measurement ranges from a few millimeters to less than one nanometer. They also have the advantage to allow measurements without contact, without degradation nor preparation of the samples, with devices that remain affordable.
It is known in particular imaging techniques based on conventional microscopy, usually in reflection, which enable surfaces and patterns to be inspected and dimensional measurements to be performed by image analysis in a plane substantially perpendicular to the observation axis. These devices usually comprise a light source, a camera and an imaging optics with a suited magnification. Their lateral resolution, in the order of one micrometer, is essentially determined by the optical diffraction phenomenon, the magnification and the quality of optics. The measurements are usually made in the visible or near ultraviolet part of the light spectrum, which enables diffraction to be restricted, and cameras and optics at a reasonable cost to be used.
For the purpose of obtaining depth quantitative measurements (parallel to the observation axis), the imaging microscopy can be complemented by interferential measurements, according to interferometric microscopy techniques. The device is then complemented by an interferometer which enables light from the surface of the object to be measured (the measurement wave) and a reference light wave from the same source and reflected by a reference surface to be superimposed on the camera. Interferences are thus obtained between the measurement and reference waves which enable the topology of the surface to be measured with a depth resolution in the order of one nanometer. For implementation reasons similar to the case of imaging microscopy, measurements are usually made in the visible part of the light spectrum.
The interferometric microscopy enables, for example, topography measurements on a first surface, or thickness measurements of thin layers substantially transparent to the wavelengths used, to be effectively made. On the other hand, it can hardly make thickness measurements of materials higher than a few tens of microns without optical compensations delicate to implement, and of course it does not enable silicon thicknesses to be measured, given that this material is not transparent to visible wavelengths.
The issue in measuring thicknesses is effectively solved by interferometric measurement techniques, in particular based on low-coherence infrared interferometry. Indeed, a number of materials widely used in microelectronics and in microsystems such as silicon or gallium arsenide are substantially transparent for wavelengths in near infrared. These are generally point measurement systems, namely capable of measuring one or more heights or thicknesses (in the case of measurements on stacks of layers) at a point of the surface of the object.
Another issue in microsystems and in microelectronics is the measurement of heights of patterns having a high depth to width ratio (also called “aspect ratio”). These patterns, made in particular by deep plasma etching (“Deep RIE”) can have, for example, the form of trenches or holes a few micrometers wide and a few tens of micrometers deep. The measurement of their depth is particularly difficult exactly because of the aspect ratio. All the techniques based on an optical measurement beam which has a high numerical aperture, which includes imaging based techniques, whether interferometric or not, and confocal techniques, are inoperative because the beam cannot reach the bottom of the structures under usable conditions.
Document FR 2 892 188 to Courteville describes a method and a device capable of measuring the height of patterns which have a high aspect ratio. The device includes a substantially punctual measurement beam, which covers a restricted region on the surface of the object. The height measurement of patterns covered by the beam is obtained by dividing the incident wave front between the high and low parts of patterns and interferometrically measuring phase shifts induced between these fractions of wave fronts after a modal filtering step. The device described in FR 2 892 188 can advantageously be implemented at infrared wavelengths to simultaneously measure thicknesses of layers of semi-conductor materials.
Characterizing elements in microelectronics or in microsystems often requires simultaneously topology measurements and height or thickness measurements performed in particular places. Locating these height or thickness measurements should sometimes be very accurate, for example in “chip level packaging” applications where apertures or vias a few micrometers wide spaced apart by several tens or hundreds of micrometers are pierced through the semi-conductor substrate. In other cases, height and thickness measurements should be performed in a region having a restricted range to take only some patterns into account. In all cases, the infrared measurement beam should thus be accurately adjusted in position and/or magnification on the surface of the object.
Document FR 2 718 231 to Canteloup et al. is known which describes a height or thickness measurement method using a point measurement beam the position of which is viewed on a camera. The measurement beam passes through the imaging optics of the camera such as to appear in the field being viewed. This device enables the measurement beam to be accurately positioned on the surface of the object. However, the wavelength of the interferometric measurement is in this case included in imaging wavelengths for which the imaging optics is optimized. This is a strong constraint in the implementation described in FR 2 718 231, related in particular to the fact that interferometric measurement techniques are mostly very sensitive to parasitic reflections, multiple optical paths and other aberrations of wave fronts which inevitably appear when an optics is not optimized for the operating wavelength. In particular, the method described in FR 2 718 231 is unsuitable for an interferometric measurement system in the infrared.
The purpose of the present invention is to provide a device for inspecting structured objects, capable of simultaneously producing topography measurements, layer thickness and pattern height measurements.